A distributed antenna system (“DAS”) can be used to extend the coverage of a cellular communication system. For example, a DAS can extend coverage to areas of traditionally low signal coverage within buildings, tunnels, or in areas obstructed by terrain features.
A DAS can include one or more master units in communication with carrier systems, such as base transceiver stations of cellular service providers. The DAS can also include remote antenna units physically separated from the master unit, but in communication with the master unit via a serial link that may be copper, optical, or other suitable communication medium. The remote antenna units can wirelessly communicate with user devices positioned in a coverage area.
For example, the remote antenna units can be positioned in a building, tunnel, or other structure that prevents or limits communications directly with the carriers. Remote antenna units amplify downlink signals received from the base station via a master unit and radiate the downlink signal using an antenna. An antenna unit recovers uplink signals from mobile user equipment and provides the uplink signals to the master unit. The uplink signals are summed together and provided back to the base station.
A remote antenna unit typically includes at least one duplexer for separating uplink signals and downlink signals. Duplexers isolate a transmitter output from a receiver input by allowing frequencies within the downlink band to be provided from the transmitter output to the antenna and allowing frequencies within the uplink band to be provided from the antenna output to the receiver. Isolating a transmitter output from a receiver input prevents downlink signals from interfering with uplink signals. Isolating a transmitter output from a receiver input also prevents the receiver from recovering transmitter-generated noise that would desensitize the receiver.
Duplexers, however, are undesirable for a variety of reasons. Duplexers use fixed filters tuned to the specific frequencies. A DAS covers a wide range of frequencies for flexibility and cost reduction reasons. The allocation of these frequencies into bands may change over time and are typically different in different countries. Re-tuning a duplexer involves a multi-step tuning procedure to change the position of a multitude of tuning screws. Re-tuning a ceramic duplexer may involve the use of a hand tool to re-shape the duplexer. The manual configurations use a network analyzer to identify the resulting change in the operation of the duplexer. Duplexers using fixed filters provide little or no flexibility to respond to changes in frequency band allocation.
One solution for isolating uplink signals from downlink signals without a duplexer is an RF-impermeable layer separating transmit and receive antennas. This solution is generally sufficient to prevent transmitter-generated noise from desensitizing the receiver, but additional isolation implemented with or without the RF impermeable layer may be desired. Therefore, systems and methods are desirable that provide additional signal isolation without the use of a duplexer.